Many automatic transmissions include a throttle valve cable ("T.V. cable") that forms a mechanical link between the motor vehicle fuel delivery system and the transmission. A throttle lever and bracket assembly transfers the movements of the throttle plates in the fuel delivery system (such as a carburetor or an electronic fuel injector) to a throttle valve plunger in a throttle valve valve body. Throttle valve pressure and line pressure increase according to the throttle opening. The throttle lever and bracket assembly also control modulated and detent downshifts.
If the throttle valve ("T.V. cable") is properly adjusted, the throttle valve plunger is fully depressed when the engine is at wide open throttle. With proper adjustment of the T.V. cable, the movement of the throttle valve plunger in the throttle valve valve body is calibrated to the movement of the fuel delivery system. The transmission should have the correct oil pressure and shift feel regardless of engine torque. When properly calibrated, the engine torque and transmission line pressure/shift feel should always be in balance.
Factory settings for the throttle valve cable usually require that (1) the distance between the distal end of the T.V. cable be at most about 1.125 inches from the pivot point of the throttle shaft linkage, and (2) the distal end of the T.V. cable travels no more than an angle of about 78 degrees (measured from the pivot point of the throttle shaft linkage) from closed throttle to wide open throttle. The proximal end of the T.V. cable is located closer to the throttle pedal. On some vehicles, the distance from the pivot point to the attachment point for the T.V. cable is greater than the recommended setting. In others, the angle of travel is greater. In each instance, the T.V. cable will be longer than recommended at wide open throttle.
The length of the throttle valve cable is critical to proper transmission shift performance. If the T.V. cable is shortened by moving it towards an adjuster tab on the T.V. cable housing, both line pressure and shift points will be raised. However, the effect is usually only temporary. When the driver depresses the throttle to the full throttle position, the T.V. cable will readjust to its approximate normal position and length. The T.V. cable adjuster will ratchet out when subject to a pulling force of about 18 pounds.
Alternatively, if the throttle valve cable is lengthened, both line pressure and shift points are lowered. Under this lengthened condition, the T.V. cable will not readjust when the throttle is depressed to the full throttle position. Thus, the effect is to allow engine torque to be ahead of transmission line pressure and shift points. Early shifts, slipping and/or no detent downshifts may occur. In addition, transmission line pressure will be low. Operating the vehicle with a longer T.V. cable causes premature clutch wear and other transmission malfunctions.
Many vehicle owners would like to defeat the self-adjusting throttle valve cable and tailor shift points or shift feel. However, on most vehicles, and especially on a vehicle with a GENERAL MOTORS THM 700-R4 automatic transmission, the T.V. cable controls both shift feel (clutch apply pressure) and shift points. One cannot alter one without altering the other. A self adjusting T.V. cable that could be set to a predetermined shorter length to raise line pressure and shift points, but that could also "adjust" itself to a longer length when the vehicle is operated at full throttle would solve these problems. Accordingly, it is an object of the present invention to have a method and an apparatus to adjust the shift performance of an automatic transmission without adjusting the predetermined length setting of the T.V. cable.